1. Field of the Invention
The invention relates to the maintenance of boilers in electric power generating plants and concerns measurement of boiler tube wall thickness for detection of areas of reduced wall thickness. More particularly, the invention relates to a portable, hand-held tube wall thickness measurement scanning probe for ultrasonic inspection of boiler tubes and to the monitoring and analysis of tube wall thickness measurements.
2. Description of the Related Art
Boiler tube failure is a major problem in fossil-fired electric power generating plants. These boiler tube failures commonly occur as the result of erosion and/or corrosion of the metal at the inner surfaces of boiler tube walls. Many of these tube failures are of boiler tubes which constitute the waterwalls of boilers, i.e., the outer walls of the boiler that enclose the furnace.
A large majority of waterwall tube failures are attributable to the localized effect of hydrogen damage, the root cause of which is believed to be salt in-leakage through deteriorating condensers. Replacement of condensers cannot solve the problem of existing damage in power generating plants, many of which have been in operation for decades. Damaged tubes must be found in order to remove them from service before failures occur.
The condition of the boiler tubes can be tested by the use of conventional ultrasonic thickness gauges to determine whether, and at what locations, significant tube wall thickness reduction has occurred. Such testing allows repair or replacement of damaged boiler tubes before holes form in the tube walls.
The testing of boiler tubes with presently available ultrasonic thickness gauges is a labor-intensive and time-consuming procedure which is carried out, for the most part, during power outages, and is therefore limited to the examination of only a very small portion of the total wall surface area of a boiler. A typical boiler of a mid-size (450 MW) turbine-generator may contain over 50 miles of waterwall tubing. Because of the large amount of tubing to be inspected and possibly replaced, and the limited time that it is economically feasible for a boiler to be out of service, it has not been possible to prevent boiler tube failure by routine inspection of boiler tubes with conventional ultrasonic thickness gauges. Recent attempts to make boiler tube inspection faster and easier include the use of ultrasonic scanning probes that provide wall thickness measurement information to data processing units.
Michael J. Nugent and Shannon J. Hansen, "Automated Boiler Waterwall Tube Inspection for Hydrogen Damage", paper presented to the American Society for Nondestructive Testing (ASNT), Fall Conference, November 1993, describes a semiautomated robotic inspection device for locating areas of hydrogen damage in boiler tubing developed and used by Con Edison Company of New York, Inc. The paper describes a motor-driven crawler employing permanent magnets to attach itself to the furnace wall, and having nine dual-element ultrasonic transducers for simultaneous testing of three parallel tubes.
D. M. Schlader et al., "EPRI Portable EMAT Wall Thickness Measurement System", paper presented to the Electric Power Research Institute (EPRI) 4th Conference on Fossil Plant Inspections, San Antonio, Tex., Jan. 18-20, 1994, describes a boiler tube thickness gauge that employs electromagnetic acoustic transducers that do not use a fluid couplant as required for other kinds of ultrasonic testing. A hand-held probe for testing one tube at a time is described.